President Reagan's triumph on the MX missile proved beyond a doubt that the arms race is not going to be stopped in Washington.

But out in the country, several people are trying to stop it in other ways.

You may remember David Mixner. In the summer of 1969, the first year of Richard M. Nixon's presidency, he and three of his buddies, Sam Brown, David Hawk and Marge Sklenkar, organized the Vietnam Moratorium, which brought thousands into the streets to demand "Peace now."

His elders scolded him, warned of the unwisdom of "unmaking" another president, told him, "Richard Nixon knows he has to end the war." Mixner and company were, however, right. For Nixon, mere peace was not enough -- he had to have a generation of it, and kept the war going for 4 1/2 more years.

Vice President Spiro T. Agnew, the Nixon administration's hatchetman until the law overtook him, immortalized the moratorium's organizers as "an effete corps of impudent snobs."

Now, faced with a president who contends that reducing nuclear weapons is not enough -- he wants them eliminated -- Mixner is taking to the streets again, or, to be more precise, hitting the road. He is organizing "The Great Peace March," a 3,235-mile trek, to begin in Los Angeles next March 1 and end in Washington eight months later. He expects 5,000 Americans to leave their schools, jobs and families to walk through deserts and blizzards to show commitment to peace.

Since those years when he so grievously irritated Nixon, Mixner has prospered -- and is just as rosy-cheeked, sunny and overweight as he was then. He founded a highly successful political consulting firm in Los Angeles and became a gay rights activist.

Now 37, he has given his share of his company to his employes and is deep in planning the logistics of the peace march, recruiting regional coordinators, choosing backpacks, testing tents, sending application blanks to marchers.

He anticipates no difficulty in recruiting 5,000 marchers. The tents have little porches, so that the marchers can entertain citizens of the communities they pass through and tell them about Phase II, a massive civil disobedience campaign, to be conducted at selected weapons sites and calculated to clog the courts and jails for months.

Mixner does not need to be reminded of the ease with which Reagan crushed the nuclear freeze, which three years ago swept the country and looked like an irresistible political force. He thinks the trouble was that the freeze asked nothing of its adherents.

ProPeace, the name of the marching organization, does. Mixner will be asking people to give up a year of their lives to dramatize their commitment. They will be asked to trudge through rain and snow, to remain "cheerful and friendly for 255 days" and spread the gospel of peace.

Mixner can already hear what will be thrown at him by the Reagan loyalists: "nostalgia trip," "the return of the '60s," "crazys," "gypsies" and surely, the way things go now, "traitors."

"None of that bothers me," he says. "All I would mind is if people have lost the belief that they can do something about the nuclear arms race."

Mixner wants to raise $1 million by June 1. He has a big staff, some of whom are scouting campsites in 16 states. Thousands of applications are in the mail to would-be marchers. Riffraff looking for three squares and a little company will be weeded out in a month-long shakedown camp in Los Angeles. Drugs and alcohol will be prohibited.

The march is the easy part of the enterprise. Hardest will be the approach to the Eastern bloc and the Soviets, where people who demonstrate against their government have to make a greater "sacrifice" than a long walk. This part of the strategy is "the least defined," according to Mixner.

One person who is trying to slow the arms race in a less grandiose way thinks the march is a wonderful idea because people will follow it on television and have their attention focused on the issue. He is John Barlow, 37, a Wyoming rancher who lives on land his family has occupied for four generations.

Although he said that all his life he believed that "the human race was going to annihilate itself," he was only converted to activism by the birth of his daughter 2 1/2 years ago. "When I saw her," he said, "the idea became untenable."

He is president of the Wyoming Outdoor Council, joined Western Solidarity and twice has come to Washington to lobby, unsuccessfully, against the MX. Now, with 25 other environmental organizations, he is seeking an injunction against the installation of MX missiles in Wyoming, which is thickly sown with Minuteman missiles.

Their argument is that the environmental impact statement failed to take into account "the consequences of a thermonuclear exchange."